Environmental monitoring during the keel access and mercury canister retrieval operation at U-864

This report presents results from environmental monitoring conducted the 19th to the 27th of June 2026 during operations for keel access and mercury canister retrieval at the U-864 submarine wreck outside Fedje. The monitoring programme aimed to document baseline conditions and to ensure early detection of particle-bound and dissolved mercury associated with the offshore activities. A baseline survey, including turbidity measurements and water sampling, was completed during the March operation and used in this study. During the first two days of the canister retrieval operation, background blank samples were measured consistently below 5 ng L⁻¹, though due to contamination of the sample vessels, blank samples during the remaining monitoring period varied between 3 and 24 ng Hg L-1. Most (73 %) of analysed water samples still had total mercury concentrations below 50 ng L⁻¹. It was estimated a flux of 3.2 kg of Hg out of the operational area during this operation, of which around 38% was in its dissolved phase. The report concludes that the revised monitoring approach functioned technically as intended and provides recommendations for improvements to turbidity measurements, water sampling, and sediment trap handling ahead of future operations.